[The neural pathway from nucleus accumbens to amygdala in morphine analgesia of the rabbit].

The purpose of this study was to explore possible neural connections between the nucleus accumbens and amygdala involved in analgesia. The latency of the escape response elicited by radiant heat applied on snout of the rabbit was taken as an index for nociception. It was found that: (1) the analgesic effect elicited by intra-accumbens injection of morphine was attenuated dose-dependently by intra-amygdala administration of opioid antagonist naloxone, or antisera against met-enkephalin (ME) or beta-endorphin (beta-EP), suggesting the involvement of ME and beta-EP in the accumbens-amygdala connection; (2) the analgesia produced by intra-amygdaloid injection of morphine was not affected by naloxone administered to nucleus accumbens, suggesting a one-way connection between the two nuclei; (3) the analgesia of intra-accumbens injection of morphine was significantly attenuated by muscimol, the GABA receptor agonist, and enhanced by bicuculline methochloride, the GABA receptor antagonist injected into the same site where morphine was administered. The results suggest that the analgesic effect of morphine administered to the nucleus accumbens is mediated by the suppression of the GABAergic inhibitory neurons located within the nucleus.